1. Field of the Invention
The present invention relates to a shutter for a cartridge for a magnetic or optical recording medium, such as a micro-floppy disk, and further relates to a stainless steel sheet for the production of such a shutter.
2. Description of Related Art
A cartridge such as a cassette for a floppy disk generally comprises a plastic cassette for accommodating a disk and a shutter for closing and opening a slot formed in the cassette body and through which a recording and reproduction head can access. The shutter is adapted to open the slot only when the disk is in use and to close the slot when the disk is not in use, that is, when no recording or reproduction is to take place, so as to protect the disk inside the cassette.
Such a shutter is produced, in general, by stamping stainless steel into a predetermined shape, and then bending it. An alternative is to injection-mold or heat a synthetic resin or a like material to bend it into a predetermined shape.
Since a synthetic resin shutter tends to take on an electrostatic charge, there is a high risk that foreign matter, such as dust, will adhere to the magnetic recording medium, thereby causing a high risk of signal dropping, called signal dropouts. In order to prevent these risks, a synthetic resin shutter is surface coated with an antistatic agent or the like. However, since the use of a synthetic resin suffers other problems, such as insufficient heat resistance, a metal such as stainless steel has been used in many cases.
A stainless steel shutter, as illustrated in FIG. 5 of the drawings, is generally composed of a steel sheet known as a hair-line finished material prepared by melting an alloy whose chemical composition is, for example, 18% by weight (percentages by weight will hereinafter be abbreviated as "%") of chromium (Cr), and 8% of nickel (Ni), the balance essentially consisting of iron (Fe), subjecting the alloy to hot rolling, cold rolling and heat treatment so as to obtain an alloy sheet of a predetermined thickness, and forming fine streaks or lines on the surface of the sheet with emery cloth or the like so that the surface will be and remain free from marks such as fingerprints, and will thus maintain good external appearance.
In the production of such a shutter, in order to prevent curling of the stainless steel sheet material when it is being rolled, the roughnesses of hair lines on the upper and lower surfaces of the steel sheet are controlled to be the same. However, the actual functions required from a shutter basically differ as between its front surface and the reverse surface.
Therefore, when a normal hair-line finished material is used for making a shutter, the production of the shutter and the shutter itself face the following problems:
(1) When the shutter is being opened or closed, the hair-line finished inner surface of the shutter abrades a portion of the plastic cartridge body. Dust generated by abrasion may adhere to the magnetic recording medium, and may cause signal dropout;
(2) If the outer surface of the shutter does not have sufficient hardness, the outer surface of the shutter tends to become scratched during the process of assembling the cartridge. Such scratches may spoil the external appearance of the cartridge, and may decrease yield;
(3) If the hardness of the shutter material is to be increased in order to prevent formation of scratches, this makes it necessary to form the shutter material by strongly bending it, but this presents the risk of cracking portions which are to become corner portions of the shutter. In addition, stamping the shutter material becomes difficult.
(4) In order for the shutter to be able to maintain its desired configuration, the shutter material must have a certain minimum of rigidity. However, since sufficient hardness is required, as described above, it is difficult to achieve the required rigidity as well; and
(5) The shutter metal should be able to accept printing thereon, such as printing stating the characteristics, the functions and the trade mark of the cartridge. However, the use of a conventional hair-line finished metal makes it difficult to print characters and patterns having fine lines of a width of not more than 0.1 mm.